Liquid crystal display panels used in liquid crystal display devices each include a TFT substrate on which pixels having pixel electrodes and thin film transistors (TFTs) are arrayed in a matrix, a counter substrate opposing the TFT substrate and having, for example, color filters formed in positions corresponding to the pixel electrodes on the TFT substrate, and liquid crystal held between the TFT substrate and the counter substrate. In such liquid crystal display panels, light transmittance through liquid crystal molecules is controlled for each pixel to generate an image to be displayed.
Liquid crystal display devices being flat and light have been expanding their applications in various fields. In mobile phones and digital still cameras (DSCs), for example, compact liquid crystal display devices are widely used. For liquid crystal display devices, a viewing angle characteristic is an important characteristic. Changes in display brightness and chromaticity observed between when the screen of a liquid crystal display device is viewed straight from the front and when it is obliquely viewed depend on the viewing angle characteristic of the liquid crystal display device. Liquid crystal display devices of an in-plane switching (IPS) type in which liquid crystal molecules are driven by a horizontal electric field show a superior viewing angle characteristic.
While there are various IPS systems (hereinafter each referred to simply as an “IPS”), those in which liquid molecules are turned by an electric field generated, in each pixel, between a flat-plane like common electrode or pixel electrode and a comb-teeth-shaped pixel electrode or common electrode formed over the flat-plane like electrode with an insulating film formed therebetween can achieve high transmittance and, hence, most widely used currently.
In manufacturing a liquid crystal display device using the above type of IPS based on existing technology, first, TFTs are formed; the TFTs are covered with a passivation film; common electrodes or pixel electrodes as described above are formed over the passivation film; and comb-teeth-shaped pixel electrodes or common electrodes are formed over them across an insulating film as described above. To meet a request for production cost reduction, however, decreasing the number of layers, for example, of conductive films and insulating films in TFT substrates has been promoted. An example IPS using a reduced number of layers in a TFT substrate is introduced in Japanese Patent Laid-Open No. 2009-168878. In the substrate structure described in the patent literature, common electrodes are formed on a layer where gate electrodes are formed, and comb-teeth-shaped pixel electrodes are formed over the common electrodes across a gate insulating film and a protective insulating film.
Forming a TFT substrate for a liquid crystal display panel, whether employing an IPS or not, used to require five photomasks. TFT substrate structures which can be formed using four submasks are disclosed, for example, in Japanese Patent Laid-Open Nos. 2006-548917 and 2002-57343.